Japanese Patent Laid-Open Publication No. 2004-226196 discloses a conventional magnetostrictive load sensor that detects a load applied to a magnetic body magnetized by a current flowing through a coil, based on change in the magnetic permeability according to strain of the magnetic body. The magnetostrictive load sensor includes a ferromagnetic load-receiving portion that receives an external load, a coil wound around the load-receiving portion, and a ferromagnetic case that accommodates the load-receiving portion and the coil therein. The coil is held in a bobbin disposed around the load-receiving portion and containing a resin.
The load-receiving portion has a rod shape, and has a cylindrical hollow part in an axisymmetric area including the longitudinal center axis of the load-receiving portion. A rod-shaped component, such as a wire or a cable, is inserted into the hollow part. The load-receiving portion receives a load applied in the axial direction of the rod-shaped component.
When the load is applied to the load-receiving portion, the magnetic permeability of the load-receiving portion changes by the inverse magnetostriction effect, and accordingly changes the impedance of the circuit including the inductance of the coil. The magnetostrictive load sensor detects a load caused by the movement of the rod-shaped component inserted into the hollow part of the load-receiving portion, by measuring the voltage change across the coil that is caused by the change in the impedance.
Japanese Utility Model Laid-Open Publication No. 6-74942 discloses a conventional load sensor that detects a load using a strain gauge. The load sensor includes a rim fixer that is annular-shaped in the plan view, a load applicator at the center of the inside of the rim fixer, and strain generators that link the rim fixer and the load applicator. The strain gauge is adhered to each of the strain generators. Each of the strain gauges detects the load applied to the load applicator, and converts the load into an electric signal.
FIG. 29 is a schematic view of conventional load sensor 500 disclosed in Japanese Patent Laid-Open Publication No. 2003-194639. Load sensor 500 includes two cores 501, magnetostriction component 502, exciting coil 503, and detection coil 504.
Cores 501 extend in longitudinal direction D500. Core legs 501A are integrally formed in respective ends of cores 501 in longitudinal direction D500. Gaps 505 are provided between core legs 501A of one of cores 501 and core legs 501A of the other of cores 501. Magnetostriction component 502 contains a magnetic material, is cylindrical, and is held between two cores 501. Exciting coil 503 is wound around one of core legs 501A of each of cores 501. With application of AC, magnetic flux is generated between cores 501 and magnetostriction component 502. Detection coil 504 is wound around the other of core legs 501A of each of cores 501, and detects the magnetic flux that passes through each of cores 501.
An operation of load sensor 500 will be described below. An alternating-current (AC) current flowing in exciting coil 503 forms two paths, that is, magnetic path M502 of the magnetic flux that passes through core legs 501A and gaps 505, and magnetic path M503 of the magnetic flux that passes through core legs 501A, one of gaps 505, and magnetostriction component 502. When load F500 for holding down magnetostriction component 502 in the axial direction is applied to one of cores 501, load F500 is transmitted to magnetostriction component 502 through core 501, compresses magnetostriction component 502. Here, an increase in load F500 increases the magnetic resistance of magnetic path M503 and decreases the magnetic resistance of magnetic path M502. Thus, the magnetic flux that passes through magnetic path M502 increases. Accordingly, since the magnetic flux detected by detection coil 504 increases, the voltage induced by detection coil 504 increases. Load sensor 500 can calculate load F500 based on the voltage induced by detection coil 504.
The above conventional sensors have difficulties in reducing their thicknesses.